Assets by Segment (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Assets	$ 60,040.9	$ 57,307.7
Unum U.S.
Segment Reporting Information [Line Items]
Assets	26,701.7	24,876.9
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,568.3	3,386.3
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,294.7	3,047.3
Individual Disability-Closed Block
Segment Reporting Information [Line Items]
Assets	16,109.4	15,509.1
Corporate and Other
Segment Reporting Information [Line Items]
Assets	$ 10,366.8	$ 10,488.1